Male mice will be irradiated with graded doses of photon or particle irradiation. The induction of dominant lethal mutations in all spermatogenic stages will be evaluated by assessing the developmental capacity of the embryos which they sire. The rate of induction of mutations manifest both before and after implantation will be determined. Cell killing in the germinal epithelium will be determined concurrently. The particle beams used have a linear energy transfer ranging between 10 KeV/micron m and 3000 KeV/micron m and the relationship of relative biological effectiveness to linear energy transfer will be determined.